The invention relates to “active implantable medical devices” as defined by the Directive 90/385/EEC of 20 Jun. 1990 the Council of the European communities, specifically the devices that continuously monitor heart rhythm and deliver to the heart if necessary stimulation, resynchronization and/or defibrillation pulses of in cases of arrhythmia detected by the device.
The invention relates more precisely to the measurement of hemodynamic parameters.
The measurement of hemodynamic parameters is a key element of the cardiac stimulation. Hemodynamic parameters may include blood pressure measured by a pressure sensor (e.g. strain gauge), cardiac contractility (measured for example by an accelerometer) and heart volumes (measured for example by a conductance sensor in the left ventricle).
These measurements may be used to inform the stimulation device on the effect of a therapy such as cardiac stimulation or neurostimulation at the vagus nerve.
Moreover, in the context of such stimuli, it may be helpful to measure and record information on the patient's physiological condition, such as cardiac and cardiovascular activity, respiratory activity or gastric activity. This allows for application of the stimulation in synchronism with one or more parameters, and for monitoring the effectiveness of the pacing therapy.
For example, for patients with heart failure (typically too low ejection fraction of the left ventricle LVEF in the case of a dilated heart, or a reduction of cardiac volumes with constant LVEF in the case of an enlarged heart), cardiac function can be made more effective by vagus nerve stimulation. In addition, it is known that heart failure is often associated with high blood pressure.
Various solutions have been proposed for measuring hemodynamic parameters related to cardiac activity, in particular to adjust the parameters of a pacemaker.
The known use of a pressure sensor or of an accelerometer in a long-term implanted device may pose many problems, for example durability, the need for a dedicated lead, and power consumption.
As for the systems based on the measurement of the bioimpedance, they require a specific multipolar lead and must be placed in the heart or in the vicinity thereof
Another technique is described by WO 2013/022886 A1, consisting of measuring hemodynamic parameters by injecting a current in an organ of a cervical body area and the collection of changes in voltages induced in the vicinity, for example to assess instantaneous changes in the volume of an artery by impedance measurement (impedance plethysmography). In a monopolar configuration, the impedance is measured between the case of the implanted generator and an electrode placed on the vagus nerve, but this configuration does not allow obtaining a measure of the representative local impedance of blood flow in the carotid artery.
In another bipolar embodiment, these local changes can be measured, but any variations in contact impedance of the electrode at the current injection point is likely to cause a variation of voltage, not representative of blood flow. This variation is further capable of creating, in the signal voltage, jumps that may saturate the amplifier stages of the detection circuit of the device.